Author: '''Jérôme Chanut, Mercator Océan'''

Page related to ticket: #1620 and associated to the following development branch:
[​https://forge.ipsl.jussieu.fr/nemo/browser/branches/2015/dev_r5847_MERCATOR9_solveur_simplification]

This development is part of the shared simplication process, task: SIMPLIF-2: Removal of filtered free surface. Split explicit and explicit free surface only.

'''Step 1: Remove filtered free surface option and associated key_dynspg_flt key'''

 
1) Remove parts of the code specific to "key_dynspg_flt" or lk_dynspg_flt=T and the need for solver modules in the code.[[BR]]
2) Suppress SOL directory and dynspg_flt module[[BR]]

3) Clean momentum trend routines related to filtered free surface (trddyn, trdken, trd_oce).[[BR]]
4) Update reference configurations: [[BR]]
- Replace key_dynspg_flt by key_dynspg_ts if needed in CONFIG/* directories. [[BR]]
- Remove namsol block in namelist_* and references to flt variables (trends) in field_def.xml[[BR]]
- SETTE: update sette.sh, to remove modifications of namelist variables related to solver. [[BR]]

NB: one should change "solver.stat" file and numsol names in stp_ctl and in_out_manager... keep it as is for the time being[[BR]]

==> '''Commit changes to revision:5868'''
[https://forge.ipsl.jussieu.fr/nemo/changeset?old_path=%2Fbranches%2F2015%2Fdev_r5847_MERCATOR9_solveur_simplification&old=5867&new_path=%2Fbranches%2F2015%2Fdev_r5847_MERCATOR9_solveur_simplification&new=5868]

==> Pass Sette tests:YES

'''Step 2: Suppress implicit pressure gradient option, ln_hpg_imp=T '''

Since this option is not supported with the now default split explicit free surface: remove it ( => simpler step routine).
ln_hpg_imp namelist variable suppressed in reference namelist.[[BR]]

==> Commit changes to revision: ???[[BR]]
==> Pass Sette tests:???

'''Step 3: Time Stepping readibility '''[[BR]]

1)''' dynzdf'''[[BR]]
Make time stepping identical in explicit or split-explicit cases => assume that "after" velocities not tendencies are available after vertical diffusion of momentum (as for tracers) => modify dynzdf routines

2) '''step'''[[BR]]
- Remove useless conditions depending on ln_dynspg_ts in step. Group all dynamic trends at the same place (at the beginning of the time stepping). Still keep dynnxt after tracer time stepping at the end.[[BR]]
- Assuming ua/va are not used anymore during tracer computation as working arrays, remove temporary 3d arrays ua_sv, va_sv

==> Commit changes to revision: ???[[BR]]
==> Pass Sette tests:???

'''Step 4: additional simplifications'''
- Remove key_dynspg_ts and key_dynspg_exp cpp keys (create namelist block to choose between the two)
- Suppress dynspg_oce module ?
- Others ?

==> Commit changes to revision: ???[[BR]]
==> Pass Sette tests:???